1. Field of the Invention
The present invention relates to a thermal head for use with a thermal printer, and more particularly to a power-thrifty thermal head having high-speed thermal responsivity.
2. Description of the Related Art
In recent years, the thermal head has been heavily used for a recording device for various information apparatuses. Thus, in order to speed up, reduce the prices of, reduce power requirements of, and miniaturize these information apparatuses, a power-thrifty thermal head having high-speed thermal responsivity has also been requested.
In order to realize such a thermal head having high-speed thermal responsivity, for heat insulating layer material, material with thermal conductivity decreased can be used, and the thickness of the heat insulating layer can be made thinner to reduce the thermal capacity. However, it is technically difficult to decrease the thermal conductivity of a glaze heat insulating layer material which has conventionally been used, and when the glaze heat insulating layer is used, the thermal capacity of the glaze heat insulating layer has been decreased by simply forming to make the thickness thereof thinner, and the reserve heat has been reduced.
For this reason, high-speed printing has generally been performed with desired thermal responsivity while sacrificing savings in electric power.
With reference to FIG. 5, the description will be made of such a conventional thermal head. On the top surface of a substrate 1 with heat dissipating property, made of alumina or the like, there is formed a glaze heat insulating layer 2 made of glass, having thermal conductivity of nearly 1.1 W/m.k.
This glaze heat insulating layer 2 is formed so as to be as thick as, for example, 200 xcexcm in thickness, and is formed with a projecting section 2a in which a portion formed with a heating element 3a to be described later protrudes at a predetermined height.
Also, on the top surface of the glaze heat insulating layer 2, there is formed a heating resistor 3 made of Taxe2x80x94SiO2, TiO2 and the like, and on the top surface of this heating resistor 3, there are formed a common power feeding member 4 and an individual power feeding member 5. In a portion sandwiched between this common power feeding member 4 and the individual power feeding member 5, there is formed a heating element 3a. 
On top of these components, a protective layer 6 made of ceramic such as Thialone is covered so as to prevent the heating element 3a or the common power feeding member 4, the individual power feeding member 5 and the like from being oxidized or worn.
In the conventional thermal head having such a structure, the individual power feeding member 5 is pulse-energized on the basis of printing information, whereby the heating element 3a is adapted to be able to selectively generate heat.
However, since such a conventional thermal head as described above has been only the glaze heat insulating layer 2 as extremely thick as, for example, 200 xcexcm in thickness formed on top of the substrate 1 with heat dissipating property, it has the same thermal conductivity.
For the reason, the exothermic temperature due to single pulse energization while the glaze heat insulating layer 2 has been cool becomes a low exothermic temperature without regard to the thickness of the glaze heat insulating layer 2. Accordingly, since great applied energy is requested during energization, there has been the problem that at the head of line at the commencement of printing, no power saving effect has been obtained, and a peak current of the battery cannot be reduced.
Also, when pulse energization is continuously performed, reserve heat in a portion in which the heating element 3a with great printing duty has been formed in the glaze heat insulating layer 2 remarkably increases because of inferior heat dissipating property of the glaze heat insulating layer 2.
For the reason, the exothermic temperature of the heating element 3 becomes excessively high, and exceeds a control range of control of energized heat, which might possibly cause deteriorated printing quality due to blotting, tailing or the like on an image printed on a recording sheet.
However, in recent years, there has been disclosed a heat insulating layer material, of which power saving does not have to be sacrificed, and which is excellent in thermal responsivity. Such heat insulating layer material is obtained by forming low oxide ceramic with low thermal conductivity by means of oxygen reactive sputtering deposition.
With reference to FIG. 6, the description will be made of a conventional thermal head using such a heat insulating layer material as described above. On the surface of a substrate 11 excellent in heat dissipating property, made of silicon or the like, there is formed a projecting section 11a having a predetermined height by means of the photolithography technique, and on top of this projecting section 11a, a heat insulating layer 12 is stacked and formed.
The heat insulating layer 12 is made up of Si, plural transition metals and oxygen, has low thermal conductivity and electrical conductivity, having thermal conductivity of nearly 0.8 W/m.k and electrical resistivity of nearly 100 xcexa9-cm, and is formed to have a thickness of 10 to 30 xcexcm on the substrate 11 by means of the oxygen reactive sputtering deposition.
Also, on the top surface of the heat insulating layer 12, in order to impart insulation characteristics and resistance to etching to the surface, an insulating layer 13 made of ceramic with insulation characteristics such as SiO2 and Al2O3 is stacked and formed in a single layer at a thickness of nearly 2 xcexcm by means of the sputtering deposition or the like.
Also, on the top surface of the insulating layer 13 as a single layer, a heating resistor 14 made of Taxe2x80x94SiO2, Tixe2x80x94SiO2 and the like is stacked by means of the sputtering deposition or the like, and a pattern of the heating resistor 14 is formed by means of the photolithography technique.
On the top surface of the heating resistor 14, a common power feeding member 15 and an individual power feeding member 16 which is made of Al, Cu and the like are formed, and a portion sandwiched between this common power feeding member 15 and the individual power feeding member 16 is formed with a heating element 14a. 
On top of these components, a protective layer 17 made of ceramic such as Thialone is covered so as to prevent the heating element 14a, the common power feeding member 15, the individual power feeding member 16 and the like from being oxidized or worn.
Since there is formed a heat insulating layer 12 of low thermal conductivity on top of a silicon substrate 11 of high thermal conductivity, the conventional thermal head having such a structure is capable of obtaining high exothermic temperature at a heating element 14a due to single pulse energization to be singly driven, and decreasing printing blurring at the head of lines where the temperature of the substrate 11 is low at the commencement of printing.
Also, peak current of the power supply is decreased and it is possible to reduce power requirements and to miniaturize the power supply. Also, on account of a combination of the substrate 11 excellent in heat dissipating property with the heat insulating layer 12 with low thermal capacity excellent in heat insulating properties due to continuous pulse energization to be continuously driven, even in continuous energization, it is possible to gently raise the temperature at the substrate 11 due to reserve heat and to provide a thermal head excellent in high-speed printing with power requirements reduced.
However, such a conventional thermal head using the heat insulating layer material as described above has had the problem that even if, on top of the heat insulating layer 12 of electrical conductivity, the heating resistor 14 and the power feeding members 15 and 16 are stacked through the insulating layer 13 made of ceramic of insulation characteristics as a single layer, current leakage occurs between each pattern through the heat insulating layer 12 because of pin hole defects peculiar to vapor deposition.
When such current leakage occurs, the thermal head may not be able to be properly operated.
Also, even if the thickness of the insulating layer 13 was increased to nearly 2 xcexcm, the leakage could not be completely eliminated. When the film thickness of the insulating layer 13 is further increased, since the insulating layer 13 had higher thermal conductivity than the heat insulating layer 12, there was the problem that thermal diffusion in the insulating layer 13 increased to deteriorate the thermal efficiency.
Further, in order to increase the thickness of the insulating layer 13 formed as a single-ply film to nearly 2 xcexcm through the use of material such as SiO2 and Al2O3 which is slow in sputtering speed, there was the problem that the production time becomes long and the productivity is deteriorated.
Also, as means for eliminating the pin hole defects peculiar to the vapor deposition, it is effective to divide and form as a film and to interpose a mechanical washing process therebetween, but there was the problem that the addition of such a washing process increased the number of man-hours required to fabricate the structure, thereby deteriorating the productivity.
The present invention has been achieved in views of such problems as described above, and is aimed to provide a thermal head suitable for high-speed printing with reduced power requirements, which eliminates leakage and prevents the thermal efficiency from being deteriorated due to thermal diffusion by forming plural layers of insulating layers of different materials on top of the heat insulating layer.
As first means for achieving the above-described object, there is provided a thermal head according to the present invention, including: a heat insulating layer formed on a top surface of a substrate; plural heating elements made up of plural heating resistors and power feeding members on the top surface of the heat insulating layer; and a protective layer for covering at least surfaces of the heating resistor and the power feeding member, wherein the structure is arranged such that the heat insulating layer is made up of an electrically-conductive multiple low oxide ceramic layer of low thermal conductivity made of a chemical compound of Si, plural transition metals and oxygen, and plural layers of insulating layers with different materials are formed on top of the heat insulating layer.
Also, as second means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the insulating layer is constructed such that a first insulating layer made up of an insulated multiple high nitride ceramic layer of low thermal conductivity which is a chemical compound of at least Si, plural transition metals and nitrogen, and on top of the first insulating layer, a second insulating layer made up of a SiO2 layer of high insulation characteristics or an Al2O3 layer are stacked and formed.
Further, as third means for achieving the above-described object, there is provided a thermal head according to the present invention, including: a heat insulating layer formed on a top surface of a substrate; plural heating elements formed of plural heating resistors and power feeding members on the top surface of the heat insulating layer; and a protective layer for covering at least surfaces of the heating resistors and the power feeding members, wherein the structure is arranged such that the substrate is made of glass whose composition is phase-separated by means of heat treatment and caused to protrude at a predetermine height at a position where the heating element is formed to form a projecting section; and the heat insulating layer is made into a two-layer structure of: a first heat insulating layer made up of a porous glass layer obtained by selectively eluting one composition of the glass whose phase is separated, which is partially formed on the projecting section; and a second heat insulating layer made of ceramic of low thermal conductivity stacked and formed on the first heat insulating layer.
Further, as fourth means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the structure is arranged such that there is formed a mask layer for covering the top surface of the substrate including the projecting section, wherein the mask layer is formed with an opening for partially forming the first heat insulating layer at an apex of the projecting section, and wherein the apex of the projecting section is exposed from this opening.
Further, as fifth means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the structure is arranged such that the mask layer is made of ceramic of insulation characteristics having a thickness of 0.01 to 0.1 xcexcm, that the first heat insulating layer is partially formed from the opening, and that the second heat insulating layer is formed on the top surfaces of the first heat insulating layer and the mask layer.
Further, as sixth means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the structure is arranged such that the first heat insulating layer is made up of a porous silicon layer of high heat insulating properties, having thermal conductivity of 0.3 to 0.5 W/m.k.
Further, as seventh means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the structure is arranged such that the second heat insulating layer is made up of a ceramic layer of low thermal conductivity having thermal conductivity of 0.8 to 1.0 W/m.k, made of a chemical compound of Si, plural transition metals and oxygen.
Further, as eighth means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the second heat insulating layer is constructed to become flat by eliminating a difference in level corresponding to a thickness of the mask layer which is caused by following a shape of the opening.
Further, as ninth means for achieving the above-described object, there is provided a thermal head according to the present invention, wherein the second heat insulating layer is constructed by stacking at a thickness of 10 to 30 xcexcm on top of the first heat insulating layer.